Question: Luis is 3 times as old as Jessica and is also 10 years older than Jessica. How old is Jessica?
Explanation: We can use the given information to write down two equations that describe the ages of Luis and Jessica. Let Luis's current age be $l$ and Jessica's current age be $j$ $l = 3j$ $l = j + 10$ Now we have two independent equations, and we can solve for our two unknowns. Since we are looking for $j$ , and both of our equations have $l$ alone on one side, this is a convenient time to use elimination. Subtracting the second equation from the first equation, we get: $0 =$ $3j$ $-$ $ (j + 10)$ which combines the information about $j$ from both of our original equations. Solving for $j$ , we get: $2 j = 10$ $j = 5$.